Apparatus for coating of a semiconductor wafer

ABSTRACT

Disclosed is a method and apparatus for coating liquid films on to the surface of a wafer substrate by rotation the substrate at a speed sufficient to cause a liquid, through centrifugal effect, to flow outwardly toward the perimeter of the surface and form a substantially uniform thickness liquid coating thereon and starting at the central region of the wafer surface and moving radially outward therefrom, spraying a fine mist of the liquid to the surface of the wafer.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.09/292,081, filed Apr. 14, 1999; now U.S. Pat. No. 6,423,380 which is acontinuation of U.S. application Ser. No. 08/949,072, filed Oct. 10,1997 and issued as U.S. Pat. No. 5,902,399; which is a continuation ofU.S. application Ser. No. 08/508,051, filed Jul. 27, 1995, and nowabandoned.

TECHNICAL FIELD

The present invention relates to an apparatus and process for coatingsolutions on to the surface of semiconductor wafers.

BACKGROUND

The current practice in semiconductor manufacturing is to use thin filmfabrication techniques. A large variety of materials can be depositedusing thin films, including metals, semiconductors, insulators and thelike. The composition and uniformity of these thin layers must bestrictly controlled to facilitate etching of submicron features. Many ofthese materials are best suited to application of the substrate with aliquid which is then dried to form the solid thin film. The liquidmaterials are most often deposited using either spin or spray coatingmethods.

In a conventional spin coating process the semiconductor wafer to beprocessed is placed on a rotatable chuck and held in place by vacuum.The chuck is referred to by a variety of names, including spin chuck andvacuum chuck. The spin chuck has a diameter slightly less than that ofthe semiconductor wafer. The wafer is positioned on the chuck such thatit is resting in a level horizontal plane with the inactive surface,designated as the bottom, in contact with the chuck and the opposite topsurface is coated with the desired solution. In standard systems thechuck is powered and rotated by a motor.

In the spin coating process, the solution can be dispensed prior tospinning the wafer, which is referred to as static dispense or after thesemiconductor wafer has been set in motion, which is termed dynamicdispense. In either case after the solution has been dispensed onto thetop surface, the wafer is spun at a constant speed to establish adesired relatively uniform thickness of the solution across the wafer.Once the liquid layer acquires the relatively uniform and symmetricalprofile, the remainder of the spin cycle allows the solvent in thesolution to evaporate to produce a solid film on the wafer top surface.

The supply of the solution is dispensed onto the wafer from a supplynozzle. The nozzle can either be configured to simply drop a specificquantity onto the semiconductor wafer surface in the form of a puddle orto spray the desired quantity onto the wafer surface in the form of amist.

After the solution is dispensed onto the wafer it is distributeduniformly over the surface largely as a result of the radialdistribution of the liquid due to centrifugal and drag forces created bythe spinning of the wafer.

The solution deposited on the wafer goes through a number of stagesduring the spin process, primarily due to the fluid dynamics created bythe spinning substrate. At the start of the spinning a wave of solutionis created that moves towards the edge of the wafer. As the majorportion of the solution in the wave reaches the edge of the wafer itforms a ridge, this is referred to as the corona stage. A beadsubsequently forms along the edge of the wafer as the solvent evaporatesfrom the ridge formed in the corona stage. As the corona disappears theremaining solution leaves the surface in the form of a fine spiral-likemist. This spiral stage results in thousands of droplets spinning offthe wafer and splashing back onto the wafer off of the surroundingspinner bowl. Bowls and splashguards have been designed to prevent thissplashing. Additionally, solvent washing of the bottom of the wafer caneliminate the edge bead that forms during the spin coating.

Every layer deposited on the top surface of the wafer that possessesirregularities and variations in its uniformity has an adverse affectduring all subsequent processing steps that the wafer undergoes.Uniformity of the layers is a critical factor in semiconductor waferproduction. The film thickness uniformity obtained using the spincoating process is largely a function of size and shape of the wafer,because of the influence centrifugal force has in the spin coat process.The fluid dynamics described above become more pronounced as thediameter of the wafer used increases and the trend is towards usinglarger wafers.

To compensate for these undesirable influences the standard practice isto use a large starting volume of solution in the spin coating process.A large starting volume of solution also translates into a large amountof wasted solution. The increased amount of solution used also means anincrease in the cost of production for semiconductor wafers.Approximately 30-90% of the process solution used in the spin coatingprocess is wasted in the form of excess solution that is thrown off ofthe wafer substrate. The excess solution is deposited to assure a thinuniform layer in the end product.

There have been a number of inventions proposed to alleviate theseproblems. U.S. Pat. No. 5,395,649 to Ikeda, employs a plate positionedabove the wafer to change the air turbulence and fluid behavior on thewafer for improved layer uniformity.

In U.S. Pat. No. 5,405,813 to Rodrigues, a plurality of rotationalspeeds are used to increase layer uniformity and decrease the amount ofstarting solution required.

A number of patents use different types of nozzle mechanisms. U.S. Pat.No. 5,405,443 to Akimoto et al., discloses a nozzle that dispenses afixed quantity of solution without entrainment of bubbles and particlesutilizing a negative pressure system. U.S. Pat. No. 4,267,212 toShinichi, includes moving the conventional spin coat nozzle across theradius of the wafer during solution application while rotating the waferat a first and second speed. U.S. Pat. No. 5,403,617 to Haaland, enlistsa computer controlled droplet generator to select droplet size andvelocity to cause impact with the wafer without splashing. There isstill a demand in the semiconductor wafer manufacturing industry formore economical means of solution application to the wafer that improvesthe uniformity of the process layers on the wafers and uses lesschemicals.

The other major process used to deposit dielectrics on to wafersubstrates is a spray coat process. The spray coating process permitsmuch more efficient use of the process solution because the largestarting excess needed for the spin coating process is not needed forthe spray coating process. The problem with conventional spray coatprocesses are that they require thorough and comprehensive optimizationof the process to obtain the quality of uniform layer thickness that ismore easily obtained with the spin coat process. With the spray coatprocess the size and shape of the wafer have little effect on the endresult. The uniformity of thickness of dielectric coating obtained usingthe spray process is determined by the sweeping motion of the spraynozzle.

An alternative to using either the spin coat or spray coat processes andtheir inherent problems is to coat the wafers using chemical vapordeposition (CVD). The CVD process includes the following basic steps: a)a known composition of reactant and inert gases is introduced into areaction chamber; b) the gas species move to the substrate; c) thereactants are decomposed and chemically reacted at a heated surface ofthe substrate; e) the gaseous by-products are desorbed and removed fromthe reaction chamber. With the CVD process high purity films can beformed and deposited and a greater variety of starting compounds can beused. There are certain compositions that cannot be adequately appliedto the wafer by any other process. The CVD process also had certaindisadvantages. It increases both the cost of wafer production andincreases the complexity of manufacturing the wafer. There are alsooften defects in the uniformity of the layers deposited on the waferusing the CVD process. Because of the increased cost and complexity ofthe CVD process it is still used far less that either spin or spraycoating of the dielectrics onto semiconductor wafers.

SUMMARY OF THE INVENTION

The present invention is a method for coating dielectrics on the surfaceof a wafer substrate by rotating the substrate at a speed sufficient tocause a liquid, through centrifugal effect, to flow outwardly toward theperimeter of the surface and form a substantially uniform thicknessliquid coating thereon and starting at the central region of the wafersurface and moving radially outward therefrom, spraying a fine mist ofthe liquid dielectric to the surface of the wafer.

An advantage of the invention is that the amount of liquid that iswasted as excess is reduced from as much as 90% to 10%. Anotheradvantage of the present invention is that less organic solvent is usedbecause less total material is required to coat the wafersatisfactorily. Decreased use of organic solvent improves the indoor airquality of the workplace as well as the safety and overall improvementof the environment, over which their is increasing concern and stricterregulation. The decrease in organic solvents also decreases the risk offlammability and the problems associated with proper disposal of organicsolvents. The process of the invention still allows for the decreasedamount of material associated with a spray coating process while stillyielding the layer uniformity achieved with a spin coating process. Thisprocess also does not require the comprehensive fine tuning required inthe conventional spray coating process.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description that follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a representation of a prior art spin coating device showing acoating liquid pooled at the center of a wafer;

FIG 1B is a representation of the wafer of FIG 1A with the waferspinning to form a thin coat on top of the wafer;

FIG. 2 is a representation of the prior art spray coating process; and

FIG. 3 is a representation of the spray/spin coating process andapparatus according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 are schematic representations of the prior art. FIG. 3schematically represents the wafer coating method of the presentinvention. A spin chuck 12, holds wafer 20 by vacuum to allow spinningof wafer 20. In the preferred embodiment, wafer 20 is rotated at a speedranging between 500 and 1500 rpm in an 50% relative humidity environmentat 72° F., depending the solution that is going to be applied to wafer20. Spin chuck 12 holding wafer 20 is enclosed by bowl 30 inside ahousing. Bowl 30 can be moved up or down to surround wafer 20. Drain andexhaust pipes are connected to the underside of bowl 30. The spatialrelationships and operation of spin chuck, 12, wafer 20, bowl 30 and thehousing are omitted as they are known in the art.

While wafer 20 is being rotated within the proscribed speed range nozzle40 is positioned over the center point of wafer 20 and dispensing ofsolution from nozzle 40 over already rotating wafer 20 is initiated.Nozzle 40 moves radially away from the central point of wafer 20 whilecontinuously dispensing the selected solution. The radial speed ofnozzle 40 as it moves radially away from the center point of wafer 20 isconstant. Dispensing of solution by nozzle 40 is discontinued at apredetermined position, here when nozzle 40 is within 1 cm of the outeredge of wafer 20. It is also possible that the direction of nozzle 40could be reversed and start dispensing at the outside edge of wafer 20and discontinue dispensing at the center point of wafer 20. The nozzlecan be halted at the center by fixing a nozzle-stopper in a positionabove the center of the chuck 12 and co-planar with the nozzle 40. Inanother embodiment nozzle 20 can begin dispensing at the outside edge ofwafer 20, and dispense solution across a diameter of wafer 20 anddiscontinue at the opposite outside edge of wafer 20.

Nozzle 40 dispenses solution in the form of a fine mist in a dispersedand divergent pattern. Examples of appropriate nozzles include atomizingnozzles, airless spray nozzles, and the like. Nozzle 40 is supplied froma separate pressurized reservoir.

The fine mist spray dispensing of solution of this invention allows thewafer to be spun at a moderate speed. This more moderate speed allowsless of the coating solution to be thrown off of the surface of wafer20. This method allows up to a 90% efficiency in usage of coatingsolution in the process. This greatly increases the economy ofsemiconductor wafer production. One of the greatest expenses in theproduction of semiconductor wafers is the cost of the materials thatwafer 20 must be coated with. With this method a satisfactorily uniformcoating can still be applied but with a significantly smallerconsumption of coating materials. This process works well for polymerdielectrics, but can also be used equally well for coating any solutiononto the surface of of semiconductor wafers including polysiloxanes,photo resists, developers, adhesives, protective coatings and the like.

It will therefore be understood that modifications and variations arepossible without departing from the scope of the invention as expressedin the following claims.

I claim:
 1. A spin-coat system for a semiconductor processor defining achamber and having a chuck in said chamber, comprising: a nozzle abovesaid chuck and slidably mounted to said semiconductor processor; and anozzle-stopper above said chuck, coplanar with said nozzle, and fixedlymounted to said semiconductor processor.